Currently topically applied prostaglandin analogues are the most recent innovative medications to lower intraocular pressure. Further to their powerful effect on IOP the lack of significant systemic side-effects and the once-daily dosing rapidly placed the prostaglandin analogues among the first line treatments of glaucoma and ocular hypertension. Several agents have been approved formulated as “classical” aqueous eye drops either with preservatives or preservative-free. Apparently bimatoprost shows the greatest efficiency in reducing lOP in this pharmacological class, however, among the most frequently reported common side-effects of this group like conjunctival hyperaemia and irritation, bimatoprost eye drops may have a slightly higher incidence of hyperaemia.
Bimatoprost is a prostaglandin analogue with the chemical (IUPAC) name (Z)-7-[(1R,2R,3R,5S)-3,5-dihydroxy-2-[(E,3S)-3-hydroxy-5-phenylpent-1-enyl]cyclopentyl]-N-ethylhept-5-enamide and the Formula I:

The efficiency of a product containing 0.03% by weight of bimatoprost and 0.005% by weight of benzalkonium chloride (preservative) initiated considerable efforts to improve local tolerance by reducing especially the incidence of conjunctival hyperaemia.
EP 1 753 434 discloses a composition comprising from 0.005% to 0.02% bimatoprost by weight and from 100 ppm to 250 ppm benzalkonium chloride, wherein said composition is an aqueous liquid which is formulated for topical administration to the eye.
An alternative method to improve local tolerability and reduce the incidence of topical side effects consists in applying preservative-free eye drops. EP 2 598 117 describes a preservative-free bimatoprost 0.03% solution found to be “non-inferior and equivalent” to preserved bimatoprost 0.03% with a similar safety profile of both products.
EP 2 127 638 and EP 2 178 504 disclose bimatoprost-containing formulations different from the ones of EP 2 598 117. EP 2 127 638 relates to an aqueous ophthalmic solution comprising a PGF2α analogue which solution contains non-ionic surfactant, stabilizing agent, and substantially no preservatives in a container consisting essentially of polyethylene. The PGF2α analogue is selected from the group consisting of latanoprost, isopropyl unoprostone, travoprost, bimatoprost and tafluprost.
EP 2 178 504 discloses an ophthalmic solution without antimicrobial preservative including as active substance at least one prostaglandin and a surfactant as solubilizing agent, characterized in that the solubilizer is polyoxyl-15-hydroxystearate, and characterized in that the prostaglandin concentration in the solution is between 0.02 and 1.5 g/l; The following prostaglandins are named: latanoprost, travoprost, bimatoprost, tafluprost, unoprostone. US 2011/319487 A1 discloses an ophthalmic solution whose active ingredient includes at least one prostaglandin without antimicrobial agents. Moreover, the ophthalmic solution comprises a solubilizing agent, a gelling agent, a carbomer polymerization inhibiting agent and a co-gelling/co-solubilizing agent and shows a viscosity of 8 to 20 mPa*s.
In this context the European pharmacopoeia (Ph. Eur.) rules with respect to formulating topically applied eye medicines: “Eye preparations are sterile . . . Eye-drops may contain excipients, for example, to adjust the tonicity or the viscosity of the preparation, to adjust or stabilise the pH, to increase the solubility of the active substance, or to stabilise the preparation. These substances do not adversely affect the intended medicinal action or, at the concentrations used, cause undue local irritation.
Aqueous preparations supplied in multidose containers contain a suitable antimicrobial preservative in appropriate concentration except when the preparation itself has adequate antimicrobial properties. The antimicrobial preservative chosen must be compatible with the other ingredients of the preparation and must remain effective throughout the period of time during which the eye-drops are in use.”